The present invention relates to an improvement of a speed control apparatus of an AC elevator.
The speed control system for the AC elevator include the feedback control system with a DC braking torque. That is, the high speed operation is carried out by a high speed motor and the high speed motor is disconnected from the power source when reaching to the decelerating position, and a DC current is simultaneously fed to the low speed motor by a control device having thyristors, etc., whereby the low speed motor generates the braking torque to decelerate the cage. In this case, in order to attain stable landing acccuracy and excellent confort of ride, the feedback control system is usually employed so as to generate suitable braking torque by controlling the thyristor of the controlled rectifying circuit.
This system imparts excellent characteristics of the elevator, however, the kinetic energy during the braking period is lost as heat whereby the power is lost and the temperature in the machine room is raised disadvantageously.
It has been known to decelerate the cage by the regnerative braking torque after switching to the low speed motor as the other control system for the AC elevator.
In this case, an AC power source is connected to the low speed motor to impart the regenerating braking torque in the range of higher than the synchronous speed. When the cage is decelerated to the synchronous speed of the low speed motor, the motor is switched to the power running operation to continue the low speed run to the predetermined position. At the predetermined position, the AC power source is disconnected and the cage is stopped by the electromagnetic brake.
In the system, the kinetic energy of the elevator is regenerated as the power source, whereby the saving of power can be expected. However, the torque cannot be controlled after decelerating to the synchronous speed of the low speed motor. Accordingly, it is necessary to stop it by an electromagnetic brake etc. when reaching to the predetermined position, after running it at a constant low speed. The low speed running period is a lost time for the elevator (non-control section). The time required for the operation is prolonged to remarkably decrease the function of transportation.
In the other system, th cage running by a high speed motor of a multi-speed induction motor is decelerated by regenerative braking under switching it to the low speed motor and the other braking torque such as DC braking torque is actuated before reaching to the equilibrium of the torque of the low speed motor and the load torque. The kinetic energy of the elevator is regenerated as the power source and the deceleration is imparted to save the power. On the other hand, in the low speed region of incapable of the regenerative damping, the DC braking or the electromagnetic brake etc. is actuated whereby the cage is landed at the predetermined position without long constant low speed running. However, in this system, the low speed motor is actuated only in the deceleration and the saving of the power is not satisfactory.